Zinc-based metal plated steel sheet

ABSTRACT

A zinc-based metal plated steel sheet is excellent in tribological properties during press forming. An oxide layer containing crystalline 3Zn(OH)2·ZnSO4·xH2O is formed on a plated surface. The oxide layer has a thickness of 10 nm or more. The crystalline oxide layer is composed of 3Zn(OH)2·ZnSO4·3-5H2O.

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/JP2008/066276, withan international filing date of Sep. 3, 2008 (WO 2009/031699 A1,published Mar. 12, 2009), which is based on Japanese Patent ApplicationNos. 2007-228517, filed Sep. 4, 2007, and 2008-181082, filed Jul. 11,2008, the subject matter of which is incorporated by reference.

TECHNICAL FIELD

This disclosure relates to a zinc-based metal plated steel sheetexcellent in tribological property during press forming.

BACKGROUND

Zinc-based metal plated steel sheets are widely used in many fields, inparticular, for automobile bodies. When used for automobile bodies, theyare subjected to press forming before use. Zinc-based metal plated steelsheets, however, have the disadvantage that their press formability isinferior to that of cold-rolled steel sheets. This is because in a pressdie, the friction resistance of a surface-treated steel sheet is largerthan that of a cold-rolled steel sheet. That is, the surface-treatedsteel sheet does not smoothly flow into the die at a portion of thesurface-treated steel sheet having a large friction resistance to thedie and a bead. This is liable to cause rupture of the steel sheet.

In recent years, the demand for high-tensile steel sheets has increasedto reduce the weight of automobile bodies. High-tensile steel sheetshave press formability inferior to that of mild steel sheets. Thus,high-tensile steel sheets are easily ruptured at portions ofhigh-tensile steel sheets having a large friction resistance to dies andbeads.

Galvannealed steel sheets are excellent in weldability and paintabilitycompared with galvanized steel sheets and, thus, more preferably usedfor automobile bodies.

A galvannealed steel sheet is produced as follows: a steel sheet issubjected to galvanizing and then heat treatment. As a result, analloying reaction in which Fe in the steel sheet and Zn in a platinglayer are diffused occurs, thereby forming a Fe—Zn alloy phase. TheFe—Zn alloy phase is in the form of a layer usually including a Γ phase,δ₁ phase, and a ξ phase. Hardness and a melting point tend to decreaseas the Fe concentration decreases, i.e., in a sequence of the Γphase→the δ₁ phase→the ξ phase. Thus, a high-hardness, high-meltingpoint film with high Fe concentration is effective from the viewpoint ofachieving good tribological properties because adhesion does not easilyoccur. Galvannealed steel sheets with the emphasis on press formabilityare produced in such a manner that average Fe concentrations in filmsare relatively high.

In a film with high Fe concentration, however, hard and brittle Γ phaseis readily formed at the interface between the plating film and thesteel sheet. Peeling from a surface boundary, i.e., powdering, isdisadvantageously liable to occur during processing. Thus, as shown inJapanese Unexamined Patent Application Publication No. 1-319661, for thepurpose of striking a balance between tribological properties andanti-powdering properties, a method for forming a hard Fe-based alloylayer as a second layer serving as an upper layer is employed.Disadvantageously, production by the method is costly.

As another method for improving press formability of a zinc-based metalplated steel sheet, a method for applying high-viscosity lubricant oilis widely used. In this method, however, a defect of coating due to adefect of degreasing occurs in an application step because of highviscosity of the lubricant oil. Furthermore, the lack of oil duringpress forming disadvantageously causes unstable press performance andother problems. Thus, improvement in the press formability ofgalvannealed steel sheets is strongly required.

As a method to overcome the foregoing problems, Japanese UnexaminedPatent Application Publication Nos. 53-60332 and 2-190483 each disclosea technique for improving weldability or processability by subjectingsurfaces of a zinc-based metal plated steel sheet to electrolytictreatment, immersion treatment, coating and oxidation treatment, or heattreatment to form an oxide film mainly composed of ZnO.

Japanese Unexamined Patent 4-88196 discloses a technique for improvingpress formability and chemical conversion treatability by immersingsurfaces of a zinc-based metal plated steel sheet in an aqueous solutioncontaining 5 to 60 g/L sodium phosphate and having a pH of 2 to 6,electrolytic treatment, or applying the solution described above to forman oxide film mainly composed of a P oxide.

Japanese Unexamined Patent Application Publication No. 3-191093discloses a technique for improving press formability and chemicalconversion treatability by subjecting surfaces of a zinc-based metalplated steel sheet to electrolytic treatment, immersion treatment,coating, coating and oxidation treatment, or heat treatment to form a Nioxide.

Japanese Unexamined Patent Application Publication No. 2003-306781discloses a technique for improving tribological properties by bringinga galvannealed steel sheet into contact with an acidic solution to forman oxide mainly composed of Zn on surfaces of the steel sheet andsuppress adhesion between a plating layer and a press die.

The technique for improving press formability by forming an oxide mainlycomposed of Zn on surfaces of steel sheet disclosed in JapaneseUnexamined Patent Application Publication No. 2003-306781 and the likehas the advantage over the technique using Ni and the like disclosed inJapanese Unexamined Patent Application Publication No. 3-191093 inproduction cost and environmental loading because Zn contained in theplated steel sheet is mainly used. In the case where the steel sheet isused for a difficult-to-form component, however, a high degree of pressformability is required, so that further improvement in tribologicalproperty may be required.

It could therefore be helpful to provide a zinc-based metal plated steelsheet excellent in tribological properties during press forming comparedwith the technique for improving press formability by forming an oxidemainly composed of Zn on surfaces of a steel sheet.

SUMMARY

We conducted studies on tribological properties of a galvannealed steelsheet and found the following:

-   -   A flat portion on a surface of the galvannealed steel sheet is        present as a projection compared with a surrounding portion. The        flat portion mainly comes into actual contact with a press die        during press forming. Thus, the presence of an oxide layer        containing crystalline 3Zn(OH)₂·ZnSO₄·3-5H₂O in the flat portion        prevents adhesion between a plating layer and the die. In        addition to the galvannealed steel sheet, also for a hot-dip        galvanized steel sheet and an electrogalvanized steel sheet that        are not subjected to allying treatment, the presence of an oxide        layer containing crystalline 3Zn(OH)₂·ZnSO₄·3-5H₂O on a plated        surface prevents adhesion between a plating layer and the die.

We thus provide:

-   -   (1) A zinc-based metal plated steel sheet includes an oxide        layer containing crystalline 3Zn(OH)₂·ZnSO₄·xH₂O, in which the        oxide layer is formed on a plated surface, and the oxide layer        has a thickness of 10 nm or more.    -   (2) In the zinc-based metal plated steel sheet described in item        (1), the crystalline oxide layer is composed of        3Zn(OH)₂·ZnSO₄·3-5H₂O.        -   The zinc-based metal plated steel sheet has a low friction            resistance and stably provides excellent press formability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic front view of an apparatus for measuring acoefficient of friction.

FIG. 2 is a schematic perspective view of the shape and dimensions of abead shown in FIG. 1.

FIG. 3 is a schematic perspective view of the shape and dimensions of abead shown in FIG. 1.

Reference numerals in the drawings represent the following:

1 sample used for measuring coefficient of friction 2 sample stage 3slide table 4 roller 5 slide-table support 6 bead 7 first load cell 8second load cell 9 rail N pressing load F friction resistance P tensileload

DETAILED DESCRIPTION

Our galvannealed steel sheets have an uneven surface due to thedifference in reactivity at the interface between the steel sheet and aplating film. However, the planarization of a surface layer by a methodsuch as skin pass rolling reduces irregularities of a plated surface.Thus, a force required to press projections on the plated surface with adie during press forming can be reduced to improve tribologicalproperties.

A flat portion on a surface of a galvannealed steel sheet is a portionwith which a die comes into direct contact during press forming. Thus,the presence of a hard and high-melting-point material that preventsadhesion to the die is important in improving tribological properties.Also in a hot-dip galvanized steel sheet and an electrogalvanized steelsheet which have surface irregularities smaller than those of thegalvannealed steel sheet, each of their surfaces is naturally a portionwith which a die comes into direct contact during press forming. Thus,the presence of a hard and high-melting-point material in their surfacelayers is important for improving tribological properties.

Also from this point of view, the formation of an oxide layer on asurface layer is effective in improving tribological properties. Anoxide layer containing crystalline 3Zn(OH)₂·ZnSO₄·xH₂O is veryeffective. In particular, an oxide layer containing crystalline3Zn(OH)₂·ZnSO₄·3-5H₂O is significantly effective.

Whether crystalline 3Zn(OH)₂·ZnSO₄·3-5H₂O is present in the oxide layeror not was determined by measuring an X-ray diffraction pattern of theoxide layer using X-ray diffractometry for a thin film and checking theresulting pattern against a standard pattern described in an ICDD card.The results demonstrated that peaks originating from oxides wereobserved at a diffraction angle (2θ) of about 8° to about 12° and thatthese peaks were assigned to 3Zn(OH)₂·ZnSO₄·3H₂O (ICDD card: 39-689),3Zn(OH)₂·ZnSO₄.4H₂O (ICDD card: 44-673), and 3Zn(OH)₂·ZnSO₄.5H₂O (ICDDcard: 39-688), which are trihydrate, tetrahydrate, and pentahydrate,respectively.

A thickness of the oxide layer on the surface plating layer of 10 nm ormore results in a zinc-based metal plated steel sheet having goodtribological properties. A thickness of 20 nm or more is more effective.This is because the oxide layer remains even if the oxide layer on thesurface layer is worn in press forming in which the contact area betweena die and a workpiece is large, thus not leading to a reduction intribological properties. On the other hand, the upper limit of thethickness is not set. A thickness exceeding 200 nm results in areduction in etch rate with a chemical conversion treatment liquid evenwhen the oxide layer has Zn—OH bonds, thus leading to difficulty in theformation of a dense, uniform chemical conversion film. The thickness istherefore desirably 200 nm or less.

The most effective method for forming an oxide layer containingcrystalline 3Zn(OH)₂·ZnSO₄·3-5H₂O on a surface of a zinc-based metalplated steel sheet uses a reaction with an aqueous solution. Inparticular, a liquid film of a solution containing Zn ions and sulfateions is formed on a surface of a steel sheet and allowed to stand for apredetermined time, thereby forming the oxide layer containingcrystalline 3Zn(OH)₂·ZnSO₄·3-5H₂O on the surface. In the case of using asolution containing only Zn ions, crystalline 3Zn(OH)₂·ZnSO₄·3-5H₂O isnot formed. In the case of using the solution containing Zn ions andsulfate ions, a higher sulfate ion concentration results in a tendencyto promote the formation of crystalline 3Zn(OH)₂·ZnSO₄·3-5H₂O.Furthermore, higher concentrations of Zn ions and sulfate ions resultsin a tendency to form an oxide film having a larger thickness.

The coating weight of a zinc-based metal plated steel sheet including anoxide layer on a surface of the sheet is preferably in the range of 20to 150 g/m² per surface. The reason for this is as follows: At an amountof the plating film of less than 20 g/m², the steel sheet has lowresistance to corrosion because of a small amount of the plating film.An amount of the plating film exceeding 150 g/m² results in sufficientresistance to corrosion but may cause peeling of the plating film duringprocessing. In particular, with respect to a galvannealed steel sheet,when galvannealing is performed in such a manner that good weldabilityand paintability, which are features of the galvannealed steel sheet,are satisfied, the formation of a Γ phase cannot be avoided at theinterface between the plating film and the steel sheet, causing peelingof the plating film, e.g., powdering.

The Fe concentration in the plating film of the galvannealed steel sheetis preferably in the range of 6% to 14% by mass. The reason for this isas follows: At an Fe concentration of less than 6% by mass, a pure Znphase (η phase) remains on the surface, so that the weldability, thepaintability, and the like cannot be satisfied. On the other hand, an Feconcentration exceeding 14% by mass results in the formation of a thickΓ phase at the interface between the plating film and the steel sheet,thereby reducing adhesion of the plating film. To control the Feconcentration within the range, it is important to allow a plating bathto contain an appropriate amount of Al. The Al concentration needs to bein the range of 0.05% to 0.40% by mass.

For a hot-dip galvanized steel sheet, it is important that a platingbath contain Al in an appropriate amount in order that a thick alloylayer is not formed at the interface between the plating film and thesteel sheet. The Al concentration needs to be in the range of 0.15% to0.40% by mass.

The proportion of the area of a flat portion with respect to a platedsurface is desirably in the range of 20% to 80%. At less than 20%, thecontact area between a die and a portion (recessed portion) except forthe flat portion is increased. With respect to the area of a portion inactual contact with the die, the proportion of the area of the flatportion where an oxide thickness can be assuredly controlled is reduced,thus reducing the effect of improving press formability. The portionexcept for the flat portion serves to hold press oil during pressforming. Thus, when the proportion of the area of the portion except forthe flat portion is less than 20% (when the proportion of the area ofthe flat portion exceeds 80%), the lack of oil can easily occur duringpress forming, so that the effect of improving press formability isreduced.

In the case of producing a galvannealed steel sheet or hot-dipgalvanized steel sheet, a plating bath needs to contain Al. However,additive element components other than Al are not particularly limited.That is, even if Pb, Sb, Si, Sn, Mg, Mn, Ni, Ti, Li, Cu, and otherelements are contained or added in addition to Al, the effect is notimpaired.

In the case of producing an electrogalvanized steel sheet, a platingbath may mainly contain zinc. The plating bath may contain other metalsand oxides as long as the effect is not impaired.

In a zinc-based metal plated steel sheet, the use of a high-tensilesteel sheet as an underlying steel sheet provides an effect such as areduction in weight and is thus preferred. For example, a conceptregarding a reduction in the weight of an automobile body is that theuse of the high-tensile steel sheet can reduce the weight of components(reduction in thickness of the sheet) while the crash performance of thebody is maintained. In general, however, press formability tends todecrease as increasing tensile strength. The high-tensile steel sheetapparently has inferior press formability. We conducted intensivestudies to improve the press formability of a high-tensile steel sheetand have found that the formation of an oxide layer containingcrystalline 3Zn(OH)₂·ZnSO₄·3-5H₂O on a surface layer extremely improvesthe press formability of the high-tensile steel sheet. This enables thehigh-tensile steel sheet to be applied to applications in which the useof the high-tensile steel sheet is difficult from the viewpoint offormability, thus achieving the effect of the reduction in weightdescribed above. The type of steel sheet is not particularly limited. Tosufficiently provide the effect of the reduction in weight, applicationto a high-tensile steel sheet having a tensile strength of 340 MPa ormore is preferred.

Our steel sheets and methods will be described in further detail byexamples.

EXAMPLES Example 1

A plating film having a coating weight of 60 g/m², an Fe concentrationof 10% by mass, and an Al concentration of 0.20% by mass was formed by acommon galvannealing process on a cold-rolled steel sheet having athickness of 0.8 mm. Then the steel sheet was subjected to skin passrolling. In this case, the proportion of the area of a flat portionvaried slightly with sampling positions but was in the range of 40% to60%.

Oxidation treatment was performed as follows: The galvannealed steelsheet was immersed in an aqueous solution containing zinc sulfateheptahydrate. The amount of a liquid film attached on a surface wascontrolled with a rubber roll so as to be 10 g/m². The resulting steelsheet was allowed to stand in air for 10 to 60 seconds, washed withwater, and dried. For comparison purposes, an aqueous solutioncontaining zinc nitrate hexahydrate and an acidic solution containingsodium acetate and ferrous sulfate were used. The temperature of allsolutions used for the treatment was set to 35° C.

Furthermore, a hot-dip galvanized steel sheet and an electrogalvanizedsteel sheet that have a thickness of 0.8 mm were prepared. A platingfilm having a coating weight of 70 g/m² was formed by a common hot-dipgalvanizing process on the hot-dip galvanized steel sheet. The resultingsteel sheet was subjected to skin pass rolling. A plating film having acoating weight of 50 g/m² was formed by a common electrogalvanizingprocess on the electrogalvanized steel sheet.

Oxidation treatment was performed as follows: Each of the hot-dipgalvanized steel sheet and the electrogalvanized steel sheet wasimmersed in an aqueous solution containing zinc sulfate heptahydrate.The amount of a liquid film attached on a surface was controlled with arubber roll so as to be 10 g/m². The resulting steel sheet was allowedto stand in air for 10 to 60 seconds, washed with water, and dried. Thetemperature of all solutions used for the treatment was set to 35° C.

The measurement of coefficients of friction and measurement ofthicknesses of oxide layers and analysis of 3Zn(OH)₂·ZnSO₄·3-5H₂O of theoxidation-treated plated steel sheets were performed as follows. Forcomparison purposes, steel sheets that were not subjected to oxidationtreatment were also studied in the same way as above.

(1) Test for Evaluating Press Formability (Test for MeasuringCoefficient of Friction)

To evaluate press formability, coefficient of friction of each of thesamples was measured. FIG. 1 is a schematic front view of an apparatusfor measuring a coefficient of friction. As shown in the figure, asample 1, taken from the steel sheet, used for measuring a coefficientof friction was fixed to a sample stage 2. The sample stage 2 was fixedto an upper surface of a slide table 3 that was movable horizontally. Aslide-table support 5 that was movable vertically was provided and hadrollers 4 in contact with a lower surface of the slide table 3. Byraising the slide-table support 5, a bead 6 imposed a pressing load N onthe sample 1 for measuring a coefficient of friction. A first load cell7 for measuring the pressing load N was attached to the slide-tablesupport 5. A second load cell 8 for measuring a friction resistance Fthat allowed the slide table 3 to move horizontally while the pressingload was being imposed on the sample was attached to an end of the slidetable 3. As lubricant oil, wash oil for press, Preton (registeredtrademark) R352L manufactured by Sugimura Chemical Industrial Co., Ltd.was applied to surfaces of the sample 1, and then the test wasperformed.

FIG. 2 is a schematic perspective view of the shape and dimensions ofthe bead used. Sliding was performed while the undersurface of the bead6 is pressed against a surface of the sample 1. With respect to theshape of the bead 6 shown in FIG. 2, the width was 10 mm, and the lengthin the sliding direction of the sample was 12 mm. Lower ends in thesliding direction were in the form of curved surfaces each having acurvature of 4.5 mmR. The undersurface of the bead against which thesample was pressed was in the form of a plane with a width of 10 mm anda length in the sliding direction of 3 mm.

The test for measuring a coefficient of friction was performed under twoconditions described below.

Condition 1

The bead shown in FIG. 2 was used. The pressing load N was set to 400kgf. The speed of movement of each sample (the speed of horizontalmovement of the slide table 3) was set to 100 cm/min.

Condition 2

The bead shown in FIG. 2 was used. The pressing load N was set to 400kgf. The speed of movement of the sample (the speed of horizontalmovement of the slide table 3) was set to 20 cm/min.

Coefficients μ of friction between the samples and the bead werecalculated using the following formula: μ=F/N.

(2) Measurement of Thickness of Oxide Layer

Measurement of the thickness of each oxide layer was performed with anX-ray fluorescence analyzer. A voltage and a current applied to a tubeduring measurement were 30 kV and 100 mA, respectively. An analyzingcrystal was set to TAP to detect the O-Kα ray. In the case of themeasurement of the O-Kα ray, intensities at the background in additionto the peak position were measured to calculate the net intensity of theO-Kα ray. An integral time at each of the peak position and thebackground was set to 20 seconds.

Silicon wafer pieces formed by cleavage and including silicon oxidefilms having a thickness of 96 nm, 54 nm, and 24 nm were placed on thesample stage together with the samples to calculate the intensity of theO-Kα ray on the basis of the silicon oxide films. A calibration curveshowing the relationship between the thickness of the oxide film and theintensity of the O-Kα ray was formed on the basis of the data. Thethickness of the oxide layer of each sample was calculated in terms ofthe thickness of the silicon oxide film.

(3) Determination of Presence of Crystalline 3Zn(OH)₂·ZnSO₄·3-5H₂O

The presence of crystalline 3Zn(OH)₂·ZnSO₄·3-5H₂O was determined by anX-ray diffractometry for a thin film. An X-ray diffraction pattern wasmeasured by a thin-film method using the Cu-Kα ray at an incident angleof 0.5°. A diffraction peak corresponding to a crystal structure of3Zn(OH)₂·ZnSO₄·3-5H₂O was observed at a diffraction angle (2θ) of about8° to about 12°.

For the galvannealed steel sheet, the presence of crystalline3Zn(OH)₂·ZnSO₄·3-5H₂O was determined on the basis of the intensity ratioof the diffraction peak to a diffraction peak that was observed at about42° and that originated from an alloy layer of iron and zinc. It wasdetermined that a film containing crystalline 3Zn(OH)₂·ZnSO₄·3-5H₂O wasformed when a peak intensity ratio, i.e., (peak intensity of3Zn(OH)₂·ZnSO₄·3-5H₂O)/(peak intensity of the alloy of iron and zinc),of 0.020 or more was obtained, wherein the peak intensities calculatedby subtracting their respective backgrounds were used.

For each of the hot-dip galvanized steel sheet and the electrogalvanizedsteel sheet, the presence of crystalline 3Zn(OH)₂·ZnSO₄·3-5H₂O wasdetermined on the basis of the intensity ratio of a diffraction peakthat corresponded to a crystal structure of 3Zn(OH)₂·ZnSO₄·3-5H₂O andthat was observed at a diffraction angle (2θ) of about 8° to about 12°to a diffraction peak that was observed at about 36° and that originatedfrom a zinc η layer. It was determined that a film containingcrystalline 3Zn(OH)₂·ZnSO₄·3-5H₂O was formed when a peak intensityratio, i.e., (peak intensity of 3Zn(OH)₂·ZnSO₄·3-5H₂O)/(peak intensityof the zinc η layer), of 0.020 or more was obtained, wherein the peakintensities calculated by subtracting their respective backgrounds wereused.

The peaks observed at a diffraction angle (2θ) of about 8° to about 12°were assigned to 3Zn(OH)₂·ZnSO₄·3H₂O (ICDD card: 39-689),3Zn(OH)₂·ZnSO₄.4H₂O (ICDD card: 44-673), and 3Zn(OH)₂·ZnSO₄.5H₂O (ICDDcard: 39-688), which are trihydrate, tetrahydrate, and pentahydrate,respectively.

Table 1 shows conditions of the oxidation treatment for the galvannealedsteel sheet and the results. Table 2 shows conditions of the oxidationtreatment for the hot-dip galvanized steel sheet and theelectrogalvanized steel sheet and the results.

TABLE 1 Thickness Time until of Peak washing oxide film Coefficient offriction intensity No. Solution used pH of solution with water (s) (nm)Condition 1 Condition 2 ratio *) Remarks 1 — — — 9 0.175 0.256 —Comparative Example 1 2 Zinc sulfate heptahydrate 5.5 10 s 28 0.1330.169 0.025 Inventive Example 1 3 20 g/l 30 s 34 0.134 0.163 0.049Inventive Example 2 4 60 s 45 0.132 0.163 0.080 Inventive Example 3 5Zinc sulfate heptahydrate 5.2 10 s 30 0.130 0.168 0.031 InventiveExample 4 6 50 g/l 30 s 36 0.129 0.166 0.054 Inventive Example 5 7 60 s52 0.126 0.158 0.107 Inventive Example 6 8 Zinc nitrate hexahydrate 5.610 s 22 0.160 0.228 — Comparative Example 2 9 20 g/L 30 s 28 0.159 0.2200.008 Comparative Example 3 10 60 s 31 0.155 0.215 0.011 ComparativeExample 4 11 Zinc nitrate hexahydrate 5.2 10 s 24 0.158 0.225 0.012Comparative Example 5 12 50 g/L 30 s 32 0.159 0.218 0.010 ComparativeExample 6 13 60 s 40 0.155 0.220 0.013 Comparative Example 7 14 Sodiumacetate 2.0 10 s 18 0.153 0.181 — Comparative Example 8 15 Ferroussulfate (pH was adjusted to 30 s 24 0.152 0.175 0.006 ComparativeExample 8 16 40 g/L each 2.0 with sulfuric 60 s 35 0.148 0.177 0.017Comparative Example 9 acid) *) No peak was observed at 8° to 12°.

TABLE 2 Time until Thickness of washing with oxide film Coefficient offriction Peak intensity No. Sample *1) Solution used pH of solutionwater (s) (nm) Condition 1 Condition 2 ratio *2) 17 GI — — — 6 0.1350.286 — 18 Zinc sulfate heptahydrate 5.2 10 s 25 0.135 0.162 0.022 19 50g/l 30 s 35 0.128 0.160 0.041 20 60 s 44 0.125 0.158 0.080 21 EG — — — 90.146 0.286 — 22 Zinc sulfate heptahydrate 5.2 10 s 27 0.135 0.169 0.02423 50 g/L 30 s 34 0.131 0.168 0.043 24 60 s 43 0.129 0.160 0.075 *1) GI:hot-dip galvanized steel sheet, EG: electrogalvanized steel sheet *2) Nopeak was observed at 8° to 12°.

The results from Tables 1 and 2 show the following.

In each of Nos. 1, 17, and 21, which were not subjected to oxidationtreatment, the oxide layer had a thickness of less than 10 nm. An oxidefilm adequate to improve tribological properties was not formed on aflat portion, thus leading to a high coefficient of friction.

In each of Nos. 2 to 7, 18 to 20, and 22 to 24, the peak intensity ratiowas 0.020 or more. The oxide layer containing crystalline3Zn(OH)₂·ZnSO₄·3-5H₂O was formed and had a thickness of 10 nm or more,so that the coefficient of friction was stabilized at a low level, thussufficiently improving tribological properties.

In each of Nos. 8 to 16, although the oxide layer having a thickness of10 nm or more was formed on a flat portion, the peak intensity ratio wasless than 0.020. Crystalline 3Zn(OH)₂·ZnSO₄·3-5H₂O was not formed. Ahigh coefficient of friction was measured. The effect of sufficientlyimproving tribological properties was not provided.

Example 2

Galvannealed steel sheets having different strength levels and eachhaving a thickness of 1.2 mm were used. Oxidation treatment wasperformed as follows: Each of the galvannealed steel sheets was immersedin an aqueous solution (pH: 5.5, temperature: 35° C.) containing zincsulfate heptahydrate (concentration: 20 g/L). The amount of a liquidfilm attached on a surface was controlled with a rubber roll so as to be10 g/m². The resulting steel sheet was allowed to stand in air for 10 to60 seconds, washed with water, and dried. Galvannealing was performed bya common alloying treatment to form a plating film having a coatingweight of 45 to 50 g/m² and an Fe concentration of 10% to 11% by mass.Then skin pass rolling was performed in such a manner that theproportion of the area of a flat portion was in the range of 40% to 60%.

The measurement of thicknesses of oxide layers and analysis of3Zn(OH)₂·ZnSO₄·3-5H₂O of the oxidation-treated galvannealed steel sheetswere performed by the procedure described in Example 1. Furthermore, themeasurement of mechanical properties and evaluation of press formabilityof the steel sheets were performed. Press formability was evaluated by atest for measuring a coefficient of friction and a stretch forming test.For comparison purposes, steel sheets that were not subjected tooxidation treatment were also studied in the same way as above.

(1) Measurement of Mechanical Property

A tensile test was performed in compliance with JIS Z2241 using No. 5test pieces according to JIS Z2201, a longitudinal direction (tensiledirection) of each of the test pieces being defined as a directionperpendicular to the rolling direction.

(2) Test for Evaluating Press Formability (Test for MeasuringCoefficient of Friction)

Coefficients of friction of samples were measured by the proceduredescribed in Example 1 under conditions 3.

Condition 3

The bead shown in FIG. 3 was used. The pressing load N was set to 400kgf. The speed of movement of each sample (the speed of horizontalmovement of the slide table 3) was set to 120 cm/min. Coefficients μ offriction between the samples and the bead were calculated using thefollowing formula: μ=F/N.

(3) Test for Evaluating Press Formability (Stretch Forming Test)

A spherical stretch forming test of each sample having a size of 200mm×200 mm was performed with a punch having a diameter of 150 mm(diameter of a die: 153 mm) to measure the maximum height of a formedportion when the rupture of the sample occurred. In this case, tosuppress the feed of the sample, a fold pressure of 100 ton was applied.As lubricant oil, wash oil for press, Preton (registered trademark)R352L manufactured by Sugimura Chemical Industrial Co., Ltd. was appliedto the sample.

Table 3 shows conditions of the oxidation treatment and the results.

TABLE 3 Time until Thickness Coefficient Stretch forming Sample washingof oxide Peak of test Height TS YS Oxidation with film intensityfriction of formed No (MPa) (MPa) El (%) treatment water (s) nm ratio *)Condition 3 portion mm Remarks 1 350 240 42 Performed 10 32 0.025 0.16045.6 Inventive Example 2 350 240 42 Not performed — 6 — 0.225 40.1Comparative Example 3 450 310 38 Performed 10 31 0.025 0.157 44.0Inventive Example 4 450 310 38 Not performed — 8 — 0.219 38.5Comparative Example 5 620 390 29 Performed 10 30 0.022 0.156 41.1Inventive Example 6 620 390 29 Not performed — 6 — 0.224 35.5Comparative Example 7 890 590 19 Performed 10 30 0.022 0.157 37.5Inventive Example 8 890 590 19 Not performed — 7 — 0.213 31.8Comparative Example 9 1060 660 15 Performed 10 31 0.024 0.159 36.2Inventive Example 10 1060 660 15 Not performed — 7 — 0.225 30.5Comparative Example 11 1500 800 13 Performed 10 30 0.022 0.155 35.3Inventive Example 12 1500 800 13 Not performed — 7 — 0.215 29.5Comparative Example 13 2000 1100 10 Performed 10 31 0.022 0.156 34.4Inventive Example 14 2000 1100 10 Not performed — 8 — 0.210 28.6Comparative Example *) No peak was observed at 8° to 12°.

The results from Table 3 show the following.

In each of the samples that were not subjected to oxidation treatment(Comparative Examples: Nos. 2, 4, 6, 8, 10, 12, and 14), the oxide layerhad a thickness of less than 10 nm. An oxide film adequate to improvetribological properties was not formed on a flat portion, thus leadingto a high coefficient of friction.

In each of the samples that were subjected to oxidation treatment(Inventive Examples: Nos. 1, 3, 5, 7, 9, 11, and 13), the peak intensityratio was 0.020 or more. The oxide layer containing crystalline3Zn(OH)₂·ZnSO₄·3-5H₂O was formed and had a thickness of 10 nm or more,so that the coefficient of friction was stabilized at a low level, thussufficiently improving tribological properties.

Comparisons between the steel sheets having the same strength level(Nos. 1 and 2, Nos. 3 and 4, Nos. 5 and 6, Nos. 7 and 8, Nos. 9 and 10,Nos. 11 and 12, and Nos. 13 and 14) showed that heights of formedportions of samples that were subjected to oxidation treatment(Inventive Example) was higher than those of samples that were notsubjected to oxidation treatment (Comparative Example) and that thesamples that were subjected to oxidation treatment had sufficientlyimproved press formability.

INDUSTRIAL APPLICABILITY

The zinc-based metal plated steel sheet is excellent in tribologicalproperties and press formability and can thus be applied in many fields,in particular, for automobile bodies.

1. A galvannealed steel sheet comprising an oxide layer containingcrystalline 3Zn(OH)₂·ZnSO₄·xH₂O, wherein the oxide layer is located on aplated surface, the oxide layer has a thickness of 10 nm to 200 nm, thegalvannealed steel sheet contains 6-14% by mass Fe and a proportion ofan area of a flat portion with respect to the plated surface is 20% to80%.
 2. The galvannealed steel sheet according to claim 1, wherein thecrystalline oxide layer is composed of 3Zn(OH)₂·ZnSO₄·3-5H₂O.
 3. Thegalvannealed steel sheet according to claim 1, having a tensile strengthof 340 MPa or more.
 4. The galvannealed steel sheet according to claim1, wherein the oxide layer consists essentially of crystalline3Zn(OH)₂·ZnSO₄·xH₂O.